I have made a short tutorial for someone who asked me about, how I draw shadow layers to give them more depth. To be still detailed, here is short explanation:
1) zero: no shadows
2) lowest layer: gradient shadows in the corners, because all things sunk in twilight seem to be deeper.
3) middle layer: shadows made by the sunlight.
4) highest layer: supporting shadows from middle layer by adding them more depth.
All layers are 100% transparency, multiply, colored in pink-purple.